Tracing the Lineage of the New H1N1 Strain

By David Brown
Washington Post Staff Writer
Monday, May 11, 2009

Once Upon a Time there was a little flu virus. It was probably born in Kansas in late 1917 or 1918, although nobody is really sure. Its name was H1N1. It grew up to be very wicked.

The story of the new strain of swine influenza now circling the world actually starts a lot farther back than the 20th century, but the year the "Spanish influenza" appeared is a good place to start.

From the second week in March 1918, when soldiers at an Army camp in Kansas began to get ill, until the final mini-waves of 1920, the Spanish flu infected about 97 percent of the people on Earth and killed at least 50 million of them.

The virus probably came from waterfowl, which carry dozens of different flu viruses. At some point, either before or after it got into human beings, the virus got into pigs, a species that can be infected by avian and human strains. It has stayed in swines ever since, and in people for almost as long.

The swine-origin influenza A (H1N1) virus circulating in Mexico, the United States and Canada, and present in two dozen other countries, is a descendant of the Spanish flu H1N1 virus. In the past 90 years, though, a lot of new blood -- metaphorically speaking -- has entered its lineage. It does not look or act much like its notorious ancestor.

This might be a good place to address this A and H and N business.

Influenza virus is part of a family called Orthomyxoviridae. There are four sub-classifications -- influenza virus A, influenza virus B, influenza virus C and thogotoviruses. It's like citrus fruit, which encompass oranges, lemons, limes, grapefruit, etc.

Influenza A and B cause illness in people; the others almost never do. There are many, many types of influenza A but only one influenza B.

The diversity of influenza A arises from variations in the two proteins on its surface, called hemagglutinin (abbreviated H or HA) and neuraminidase (N or NA). Together, the proteins make up the face that a flu virus presents to the immune system of a bird, a pig or a human being.

In this setting, the face's appearance is no small matter. The immune system's ability to recognize a virus is one of the first steps in stopping it.

One strategy involves antibodies. They attack only if they are tailor-made for the virus, which requires the immune system to get a good look at the surface proteins. The immune system can offer the best protection if it has seen the pathogen before and has the right antibodies ready.

Think of H as hair and N as nose, two features for learning and remembering the identity of a virus. In the world of influenza A, there are 15 subtypes of H (straight blond, wavy red, short black, kinky black, etc.) and nine subtypes of N (Roman, ski jump, flared, long, etc.). Each subtype is numbered -- H1N1, H3N2, H9N2 and so forth.

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